A porous glass manufactured using a phase separation phenomenon of glass has a unique porous structure that is uniformly controlled, and the pore diameter of the porous glass can be changed within a predetermined range. By using excellent features as described above, the porous glasses are expected to be used in industrial applications, such as adsorbents, microcarriers, separation membranes, and optical materials.
A conventionally known composition region which performs a phase separation contains approximately 30 to 80 percent by weight of silicon oxide, approximately 22 to 59 percent by weight of boron oxide, and approximately 7 to 15 percent by weight of sodium oxide. In general, a porous glass is obtained by performing a heat treatment on a glass body at 500 to 700 degrees Celsius to cause a phase separation and then performing acid etching (for example, NPL 1).
As for the composition of a phase-separable glass body, in order to improve the strength and chemical durability of a porous glass, some examples of a method forming a porous glass using a phase-separable glass body containing a SiO2—B2O3—Na2O glass added with Al2O3 and ZrO2 have been proposed. However, a heat treatment at 500 degrees Celsius or more is required for a phase separation treatment. In addition, in order to prevent precipitation of gel silica formed of a small amount of silica contained in a non-silica component phase in etching, an example in which an alkali component is adjusted by addition of Li has also been reported (PTL 1). However, since the phase separation excessively progresses by addition of Li, it becomes difficult to control the pore diameter by a heat treatment.
In addition, when a porous glass is used as an optical material, the pore diameter and porosity thereof are each an important parameter. By decreasing the pore diameter, scattering of internal light can be suppressed. In addition, by increasing the porosity, an apparent refractive index is decreased, and this porous glass functions as a glass having a low reflectance. Accordingly, when the pore diameter can be controlled, and the porosity can be increased, the degree of freedom of designing optical materials can be increased. Therefore, a material, the pore diameter and porosity of which can be controlled, has been desired. However, in order to increase the porosity, a heat treatment performed at a high temperature is required. Therefore, a phase-separable glass body composition which can form a porous glass having a high porosity by a heat treatment at a low temperature has been desired.